fix(auth): reject invalid challenge signatures instead of transitioning to AuthOk

server/crates/arbiter-server/tests/user_agent/auth.rs

@@ -165,3 +165,69 @@ pub async fn test_challenge_auth() {
 
     task.await.unwrap().unwrap();
 }
+
+#[tokio::test]
+#[test_log::test]
+pub async fn test_challenge_auth_rejects_invalid_signature() {
+    let db = db::create_test_pool().await;
+    let actors = GlobalActors::spawn(db.clone()).await.unwrap();
+
+    let new_key = ed25519_dalek::SigningKey::generate(&mut rand::rng());
+    let pubkey_bytes = new_key.verifying_key().to_bytes().to_vec();
+
+    // Pre-register key with key_type
+    {
+        let mut conn = db.get().await.unwrap();
+        insert_into(schema::useragent_client::table)
+            .values((
+                schema::useragent_client::public_key.eq(pubkey_bytes.clone()),
+                schema::useragent_client::key_type.eq(1i32),
+            ))
+            .execute(&mut conn)
+            .await
+            .unwrap();
+    }
+
+    let (server_transport, mut test_transport) = ChannelTransport::new();
+    let db_for_task = db.clone();
+    let task = tokio::spawn(async move {
+        let mut props = UserAgentConnection::new(db_for_task, actors);
+        auth::authenticate(&mut props, server_transport).await
+    });
+
+    test_transport
+        .send(auth::Inbound::AuthChallengeRequest {
+            pubkey: AuthPublicKey::Ed25519(new_key.verifying_key()),
+            bootstrap_token: None,
+        })
+        .await
+        .unwrap();
+
+    let response = test_transport
+        .recv()
+        .await
+        .expect("should receive challenge");
+    let challenge = match response {
+        Ok(resp) => match resp {
+            auth::Outbound::AuthChallenge { nonce } => nonce,
+            other => panic!("Expected AuthChallenge, got {other:?}"),
+        },
+        Err(err) => panic!("Expected Ok response, got Err({err:?})"),
+    };
+
+    // Sign a different challenge value so signature format is valid but verification must fail.
+    let wrong_challenge = arbiter_proto::format_challenge(challenge + 1, &pubkey_bytes);
+    let signature = new_key.sign(&wrong_challenge);
+
+    test_transport
+        .send(auth::Inbound::AuthChallengeSolution {
+            signature: signature.to_bytes().to_vec(),
+        })
+        .await
+        .unwrap();
+
+    assert!(matches!(
+        task.await.unwrap(),
+        Err(auth::Error::InvalidChallengeSolution)
+    ));
+}